1. Field of the Invention
This invention relates to the field of prosthetics. More particularly, it relates to a device that helps retain a socket on the residual limb of a below-the-knee amputee.
2. Description of the Prior Art
A supracondylar patella tendon bearing prosthesis (PTB/SC) is a well known prosthesis used by below-the-knee amputees. It includes an uppermost rim that is specifically contoured to fit over the medial femoral condyle. That bone protrudes to some extent on all non-obese people, and thus is used as a prominence from which the prosthesis may be suspended.
There are a number of problems with this well-known apparatus. Since the uppermost rim must be molded to fit over the medial femoral condyle of the patient, an exacting custom fit must be made for each patient. Moreover, even the best of fits allows the prosthesis to "piston" as the patient walks, i.e., the prosthesis rides up and down on the patient's residual limb. Perhaps even more problematic is the tendency of the prosthesis to come off when the patient's leg is bent beyond the normal bending of walking. For example, climbing a ladder with the conventional PTB/SC prosthesis may cause it to come detached because the medial femoral condyle slides out from under the molded rim of the device; when that happens, the device is no longer suspended from such bone, i.e., the socket detaches from the residual limb.
Thus, the three major problems with the PTB/SC currently in widespread use are the expense and time required to make it, the pistoning effect it exhibits, and its tendency to come off when the knee is bent beyond the design limits of the PTB/SC.
The conventional wisdom has been that all three of these problems can be solved, at least to some extent, by paying more attention to the customized fit of each patient, i.e., by trying to perfect existing techniques. According to this theory, if the perfect fit could be obtained, then the pistoning effect would be lessened and the prosthesis would be less likely to come off. However, even if the perfect fit could be obtained, it would require even more time and expense. Just as importantly, the pistoning effect and the occasional detachment would still occur from time to time.
Other approaches to the problem include establishing a vacuum in the bottom of the PTB in an attempt to reduce reliance upon the medial femoral condyle as a suspension means. That approach requires the addition of a check valve to the socket; that makes the prosthesis even more expensive.
Thus, developments in this art are leading either towards perfection of the fit of the PTB over the medial femoral condyle, or towards vacuuming techniques that eschew use of that bone altogether. Accordingly, it follows that the invention disclosed hereinafter, which relies upon neither vacuuming nor custom fitting, would not have been obvious at the time it was made to those of ordinary skill in this art.